mmc: sdhci: add platform get_max_timeout_count hook

Currently the max timeout count is hardcode to 1 << 27 for calcuate
the max_busy_timeout, however, for some platforms the max timeout
count may not be 1 << 27, e.g. i.MX uSDHC is 1 << 28.
Thus 1 << 27 is not correct for such platform.

It is also possible that other platforms may have different values.
To be flexible, we add a get_max_timeout_count hook to get the correct
maximum timeout value for these platforms.

Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Dong Aisheng <b29396@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>